Electric wave system



P 1942- L. w. HUSSEY 2,294,908

ELECTRIC WAVE SYSTEM Filed April 8, 1941 2 Sheets-Shee t 2 Has SIGNAL 2C(ARRIER E, /7

SIGNAL SIDES/1ND OUTPUT FIG. 9 FIG/0 Q f! (L2 I I /Nl/ENTOR 6 LIV/V0555)TORNEV Patented Sept. 8, 1942 UNITED STATES PATENT OFFICE 2,294,90ELECTRIC WAVE SYSTEM Luther W. Hussey, Madison,

Bell Telephone Laboratories,

N. J., assignor to Incorporated,

New York, N. Y., a corporation of New York Application April 8, 1941,Serial No. 387,434 14 Claims. (Cl. 179171.5)

work with the resistance element to compensatefor variation in itsimpedance with frequency.

Another feature comprises proportioning the circuit or mesh in which athermally sensitive variable negative resistance element is included,

such that the total effective impedance around the circuit is positive,

' In accordance with the invention a variable reon or varies with itstemperature is included in a circuit in which an audio frequency orsignal wave is to be modulated on a high frequency or carrier wave. Thevariable resistance element or thermistor is initially biased to apreassigned point on its voltage-current characteristic, and the signalwave is superposed on the biasing current. As the signal wave varies inamplitude, the current flow through the element, and, consequently, itstemperature and resistance vary. For high frequency, the elementpresents a substantially ohmic and constant resistance, but the signalwave, acting as a, .bias varying means, causes the carrier wave to workinto a resistance varying in accordance with the signal wave, therebymodulating the signal wave onto the carrier wave. Such a resistanceelementwaries in impedance with frequency. To minimize-distortion and tomaintain the impedance substantially constant with frequency,-acompensating network is associated with the variable resistance element:With a variable ne ative resistance element there may be a tendency tosinging around the circuit or mesh involving the element. This iscompensated for by including in the circuit a suitably proportionedpositive or constant resistance element.

. for a typical temperature controlled variable resistance element;

Figs. 3 and 4 show the curves of Fig. 2 to a different scale, andcharacteristic curves of a network to be associated with the variableresistance element, whereby the latters effective impedance variationwith frequency will be substantially constant;

Fig. 5 shows a modulation system embodying a single temperaturecontrolled variable resistance element or thermistor in accordance withthe invention;

' sistance element whose resistance is dependent A more completeunderstanding of the invention will be obtained from the detaileddescription that follows taken in conjunction with the appendeddrawings, wherein:

Fig. 1 shows typical static and dynamic characteristic curves for a'typical temperature controlled variable resistance element orthermistor;

Fig, 2 shows typical reactance versus frequency and resistance versusfrequency curves impressed on the resistance Figs. 6 and 7 showmodulation systems embodying a bridge arrangementof thermistors;

Fig. 8. shows a modulation system embodying a single resistance elementin accordance with the invention, and in which both the carrier wave andthe signal wave are amplified before being element;

Fig. 9 shows the equivalent electrical circuit for a thermistor; and

Fig. 10 shows the electrical circuit or network to be associated withthe thermistor to compensate for the latters impedance-frequencycharacteristic at audio frequencies.

Variable resistance elements or devices whose resistance is dependent onor sensitive to the temperature of the element are known. Such anelement may have a positive or a negative temperature coeificient ofresistance, and may have a voltage-current characteristic that isnonlinear. The temperature change in the element may result from theapplication of heat thereto from an external source, or the withdrawalof heat therefrom; or result from current flow, or change in currentflow; through the element. An element of the character described andwhose resistance at any particular moment is markedly dependent onitstemperature has come to be known, in recent years, as a thermistor.

Typical characteristic curves for a thermistor having a negativetemperature coeflicient of resistance, and comprising a thin layer of asemiconductive material, such as uranium oxide, engaged by a relativelysmall-area contact, for example, of chromium, are shown in Fig. 1. Athermistor of this type, together with others, is disclosed in the G. L.Pearson Patent No. 2,276,864 of March 17, 1942, If the thermistor issubjected to a direct current of increasing magnitude, the voltage dropacross it is found to increase to a maximum and then decrease. Curve Ais'the static voltage-current characteristic of a typical thermistor.

Dynamically, the alternating current resistance is negative in theregion beyond the voltage maxi- If a mum Em for sufficiently lowfrequencies. direct current lb, of value higher than rent correspondingto Em), is caused to flow through the thermistor, a superposedalternating current of frequency approaching zero will trace out a curveaob, approximately the static characteristic. As the frequencyincreases, the thermistor experiences increasing difficulty in changingin temperature, and, consequently, in resistance, with the increasinglyrapid change in current flow; eventually, a frequency is reached atwhich the voltage-current curve is along the ohmic resistance line cod.For low frequencies, therefore, the thermistor offers a substantiallylinear negative resistance to the current, and at high frequencies asubstantially linear positive resistance. At intermediate frequencies,the superposed alternating current will produce voltage-current curvesalong the broken lines e, f, g in the order of increasing frequency.

The characteristic of thermistors in offering a substantially linearnegative resistance to low frequencies and a substantially constantpositive resistance to high frequencies enables'the thermistor to beused for linear modulation purposes. In conventional modulators, linearmodulation is obtained by making the carrier wave amplitude much largerthan the signal wave amplitude. Then the system approximates one inwhich the resistance to the signal is varying at the frequency of thecarrier wave. Under such conditions, distortion components atfrequencies near the car-- rier and the desired side-bands areeliminated or minimized. In a thermistor modulator, the relativemagnitude of the carrier and signal wave is not of primary concern; thesignal wave works into a substantially linear negative resistance, andit varies a positive resistance facing the carrier.

With reference to Fig. 1, let the thermistor be biased sufficiently sothat its operating point is 0. As already explained, as the frequencywith which the bias is caused to vary plus and minus about the value ofbias for the point 0, is increased, the

current-voltage characteristic will vary from that of aob to that ofcod. Thus, the resistance of the thermistor for low frequency isnegative, given by do, d1 at i=io, 11:00, and the impedance of thethermistor for high frequency approaches the direct current resistanceIf a high frequency carrier wave and a low frewhere i and v are thetotal current and voltage and n 4. Neglecting thermal lag, the lowfrequency resistance is The high frequency and direct current resistance(the cur-- If signal wave currents (is) and carrier wave currents (in)are supplied to the thermistor, the high frequency voltage across thethermistor will be v=ica(i0+is) =icR0(l+is/io)"' Ro[1(1L+1)/ioi5]ie.lisl i0 The equivalent electrical network of a negative resistancethermistor, as shown in Fig. 9, is a negative resistance (R1) in serieswith an inductance (L1), the series resistance and inductance being inparallel with a positive resistance (R2). The impedance of thethermistor varies with frequency for low or audio frequencies, asillustrated by the curves of Fig. 2, showing reactance versus frequencyand resistance versus frequency characteristic curves X1 and RT for atypical thermistor. The constants of the equivalent network wereR1=13260 ohmsv R2=25000 ohms L1 =.1'l henries shown in Fig. 10, maycomprise a resistance (Re) and an inductance (Le) connected in series, acapacitance (C1) being connected in parallel with the series resistanceand inductance.

With the following constants: R3=30000 ohms, 3 11245 Ll-f and henries,the resistance versus frequency characteristic Rs, and the reactanceversus frequency characteristic XS of Figs. 3 and 4, respectively, areobtained. The characteristics of the compensating network are thenegative to a fair approximation of those of the thermistor, and theircombination Xe and Re provides a substantially constant impedance over asubstantial portion of the audio frequency range. If more exactcompensation, or a constant, pure negative resistance is desired, asomewhat more complex compensating network could be associated with thethermistor.

Fig. 5 shows a modulator circuit embodying the invention. It comprises athermistor T, a source In of high frequency or carrier wave, a seriesresonant circuit N1 resonant to the carrier wave, and a load L. Alsoconnected across the thermistor are a source ll of current for biasingthe thermistor to a desired point on its voltagecurrent characteristic,preferably on its substantially linear negative resistance portion, asource 20 of audio frequency or signal wave, and a parallel resonantcircuit N2 antiresonant to the carrier wave. The resistance R, is anohmic resistance, or one having a positive temperature coefficient ofresistance, and is proportioned so that the total effective impedancearound the mesh involving the thermistor will be positive to minimizepossibility of singing around the thermistor mesh. The network N is acompensating network of the type discussed with refezence to Fig. 10,and enables the thermistor to present a substantially constant impedanceto signal waves over a substantial portion of the audio frequency range.

The modulator circuit of Fig. 6 employs a plurality of thermistorsarranged in a bridge. The signal wave is impressed through thetransformer it across one pair of terminals of the bridge, and thecarrier wave is impressed through a second transformer it across theother pair of bridge terminals. The source ll provides biasing currentfor the thermistors. Resistance Rm is proportioned so that the total ofeffective impedance around the mesh involving the thermistors is.positive, and is by-passed to carrier by the series [inductance andcapacitance Na. A compensating network N of the type shown in Fig. 10,is also included in the circuit. The condenser C is a blockingcondenser, and the network N: comprises a resistance by-passed to theside-band, to ofier high impedance to the signal wave.

Fig. 7 shows a double balanced bridge.modulator embodying the invention.Each arm of the bridge comprises a thermistor T, and network N inseries, proportioned to compensate for the impedance-frequencycharacteristic of the thermistor, and providing a total eflectiveimpedance around the thermistor mesh that is positive, and low impedanceto the carrier wave and the modulation product's. Current for biasingthe thermistors to the desired point on their voltage-currentcharacteristics is provided by the-current source ii. The carrier waveis impressed across a pair of terminals of the bridge through thetransformer ii, the modulated carrier wave is taken oil from the otherpair of terminals of the bridge through the transformer i8, and theaudio frequency or signal wave is impressed on the modulator through thetransformer l9, the ends of the secondary winding of the latter beingconnected to the mid-points of the bridge windings of transformers ll,it. The condensers C and condensers C20 are blocking and by-passcondensers, respectively.

Fig; 8 shows a circuit-arrangement enabling amplification of both thecarrier wave and the signalwave before modulation. The thermistor T isconnected in the plate current of anamplifying electron discharge deviceV1, impedancecoupled to a succeeding electron discharge device V2. Ifthe device V1 is a high impedance tube such as a pentode, a high orderof stability is obtained because of the large series resistance in thesignal and the direct current circuit and good negative resistanceelement whose impedance varies with frequency, and a network compenbeingincluded in said circuit in serial relation to said element andcomprising parallel connected. inductance and capacity elements.

3. A circuit comprising a resistance element whose equivalent electricalcircuit is a positive resistance shunted by a negative resistance and areactance in series, and a network connected in series .with saidelement and comprising a reactance shunted by a resistance and areactance in series.

4. A circuit comprising a resistance element whose equivalent electricalcircuit is a positive resistance shunted by a negative resistance and aninductance in series, and a network connected in series with saidelement and comprising a capacitance shunted by a resistance-and aninductance in series.

'5. A circuit comprising a. mesh including a thermally sensitivenegative resistance element, and means to prevent self-oscillationaround said mesh comprising means to provide a positive total effectiveimpedance around the mesh involving said element. v

6. A modulator circuit comprising a mesh. including a thermallysensitive negative resistance element, and positive resistance means toprevent 2. A circuit comprising a thermally sensitive self-oscillationaround said mesh comprising means to provide a positive total effectiveimpedance around the mesh involving said element.

7. A modulator comprising a resistance element whose resistance changeswith its temperature, means to biac said element to a stabletemperature, a source of low frequency signal wave to vary'saidtemperature and consequently the resistance of said element, and asource of high frequency wave whose amplitude varies in accordance withth variation of said resistance when said signal wave is applied to saidresistance element.

8.ln combination, a source of high frequency electric wave, a source oflow frequency electric wave, and a resistance element whose resistanceis variable with its variation in temperature and that offers a linearresistance to said high fresignaling efiiciency without sacrifice of anygain in the amplifying device. If the. device V1 is not a high impedancedevice, it may be desirable to introduce the compensating networkdescribed with reference to Fig. 10 in series with the thermistorbetween it and the anode of the tube.

with a modulator of the type described there is no necessity forrelatively large carrier wave amplitude for linear modulation, there isonly small distortion at and around the carrier wave harmonics, andthere is the possibility of signal to side-band gain without the use ofrelatively expensive devices such as vacuum tubes. Although theinvention has been disclosed with reference to several specificembodiments, it will be understood that it is not limited thereto but isof a scope evidenced by the appended claims.

What is claimed is: l. A circuit comprising a thermally sensitivenegative resistance element whose impedance varies with'frequency, and anetwork for compensating for such impedance variation, said networkbeing included in said circuit in serial relation to said element andcomprising parallel connected positive and negative reactances.

quency wave and a difierent linear resistance to said low frequencywave.

9. In combination, a source of high frequency electric wave, a source oflow frequency electric wave, and a resistance element whose resistanceis variable with its variation in temperature and that offers a linearresistance tosaid high frequency wave and a different linear resistanceto said low frequency wave, said resistance element varying in impedancewith frequency, and means to compensate for said impedance over a substantial portion of the audio frequency range.

10. In combination, a source of high frequency electric wave, a sourceof lowfrequency electric wave, and a resistance element whose resistanceis variable with its variation in temperature and that offers a linearresistance to said high frequency wave .and a different linearresistance to said low frequency wave, said resistance element tendingto cause singing around the'circuit in which it is involved, and meanscomprising positive resistance to provide a total effective impedancethat is positive around the circuit.

11. An electric wave system comprising a reparallel connected inductanceand capacity ele- 5 ments, and a positive resistance in series withsaidresistance element; a source of high frequency electric wave connectedacross said resistance element; and a source of low frequency electricwave connected with said resistance ele- 10 ment.

12. A circuit comprising a variable resistance unit, means for applyingan audio frequency signal current to said unit, said unit having asubstantial negative temperature coefiic'ient of re-v l5 sistance, andbeing adapted to change in tem-.

perature and resistance substantially instantaneously with change in theamplitude of the signal current over substantially the entire audiofrequency range, and said unit varying in imped- 20 ing the temperatureof the unit such that the change in temperature of the lmit when thesig-.

ance to the signal current with variation in the frequency of the signalwave, current means for adjusting the temperature of the unit such thatthe change in temperature of the unit when the signal current is appliedthereto, varies the re- 25 ing included in said circuit in serialrelation with said unit and comprising parallel-connected positiveand'negative reactances.

13. A circuit, as claimed in the preceding claim, in which theelectrical analogue 01' the resistance unit is a positive resistanceshunted by a negative resistance in series with an inductance, and

in which said compensating network comprises inductance and capacitanceelements.

14. A circuit comprising a variable resistance unit, means for applyingan audio frequency signal current to said unit, said unit having a-sub-I stantial negative temperature coeflicient of resistance and beingadapted to change in temperature and resistance substantiallyinstantaneously with change in the amplitude of the signal wave oversubstantially the entire audiofrequency range, and said unit varying inimpedance to the signal current with variation in the frequency of thesignal current, current means for adjustnal current is applied thereto,varies the resistance substantially linearly above and below that forthe adjusted temperature, and a damped reactive network for compensatingfor theimpedance variation of -said unit with variation in the frequencyof the signal current.

. LUTHER W. HUSSEY.

